`field_default` needs to be set for all Sonarr download clients

Looks like https://github.com/buildarr/buildarr-sonarr/issues/50 extends to all download clients which can have a URL base set.

Future refactors, such as improvements to the Buildarr plugin API and switching to [devopsarr/sonarr-py](https://github.com/devopsarr/sonarr-py), should more permanently resolve issues like these in the future.